Kind of like how HN limits us to a handful of posts per day before the “you’re posting too much” roadblock, causing you to really think about what you have to say and whether you want to burn one of your budget on it. Usenet could have used something like that.
I think you're being a bit optimistic about physical spaces here. Rumor mills are as old as the hills. News spreads more slowly IRL, but that goes for truth as well as falsehood. You claim that misinformation spreads faster online than truth and imply this is different from IRL info, but I don't see why this should be the case. The same underlying reasons that favourite one message over another operate in both domains.
You're assuming that people will react to those signals by stopping (or not starting in the first place). Many of the most disruptive people look for those signals as their goal, because they want to be disruptive.
Think about people who will interpret any variation of "ugh" signals as "oooh, I've found the buttons to poke to get fun noises". If you want to solve this problem, that's a large part of the threat model.
Or to take another example of constraints -- if someone in a bar spreads a false rumor, that misinformation can quickly spread to all the patrons in the bar, but it's reach beyond that bar will be slow. By that time more factual information might also be circulating, and the damage of the false information blunted. Online, by contrast, misinformation spreads so much faster than factual information that it is often nearly impossible to counter.
I don't know what the solution exactly is here, but I feel that public spaces need to have more speedbumps. In the same way that people are jerks when they drive and the answer is often "less driving, and slower," I suspect that the answer to bad online social spaces is "less online, and slower," but I'm not sure what that looks like.